1. Field of the Invention
This invention relates to electric power switching apparatus and in particular to blow open moving contact assemblies for such apparatus with very high current interruption ratings.
2. Background Information
Power circuit breakers typically are used as a main breaker in a power distribution system having additional downstream branch circuit breakers. They are also used as transfer switches for switching between alternative power sources, and as network protectors in larger distribution systems. In such systems, the power circuit breaker must have sufficient withstand capability to allow a downstream breaker to respond to a fault in order to minimize the extent of the outage. However, in the instance of a very large fault, such as a fault just downstream of the power breaker, it is desirable to have the power breaker respond promptly to limit the fault current. It is known to provide a power circuit breaker with a blow open contact structure for this current limiting purpose. This blow opening is driven by the electromagnetic repulsion force on the contacts and is very fast, limiting the actual current to less than the available fault current. Using this scheme in a power breaker requires a rugged, but compact, contact assembly with many individual contact fingers for a high continuous capacity and to withstand the higher closing energy and short time ratings compared to molded case circuit breakers. The contact fingers must be capable of opening collectively within the contact carrier assembly without movement of the operating mechanism. The entire contact carrier assembly is opened by the operating mechanism during normal nonfault operation (without the spontaneous contact opening), and also in the instant after the spontaneous opening of a high current interruption. It is desirable that the contact assembly with the blow open moving contact structure can be used in place of a standard power circuit breaker assembly with few changes to the breaker design so that a high interrupting version can be offered in the same product family.
The contact fingers of the spontaneously opening contact moving structure must have some individual motion with springs to apply contact pressure, supported rigidly until the current-induced force threshold is exceeded. The blow open portion of the assembly must have low inertia and be compact for rapid motion from closed to the widest achievable contact gap. When open, the carrier assembly should maintain good dielectric strength across the contact gap and direct the arc produced gases toward the arc chute.
Finally, the contact assembly must accurately control contact location, force and opening threshold and be tolerant of manufacturing variation while being cost-effective to manufacture.